1. Field of the Invention
This invention relates to conversion lenses capable of changing the focal length of a master lens.
2. Description of the Related Art
The conventional simple methods of changing the focal length of a photographic system are to bring and take a negative lens unit into or out of axial alignment on the image side of a master lens having an aperture stop incorporated therein, or the so-called rear conversion method, and to releasably attach an afocal conversion lens to the object side of a master lens, or the so-called front conversion method. The photographic system employing the former method changes its F-number as the focal length changes. Another drawback is that the decentering tolerances should be very tight.
In the latter method, on the other hand, though the decentering tolerances may be set relatively loose, and the F-number can be maintained constant against change of the focal length, because the distance from the aperture stop in the master lens to the conversion lens, the conversion lens tends to become bulky and heavy. Such a front conversion lens is very difficult to build in the interior of the camera body. It has, therefore, been the common practice in the prior art to provide for the front conversion lens in the form of a separate accessory item.